I may be somewhat late in this discussion, but if Avast does not load at start-up, the following may help, probably for both version 7 and 8 en probably also for all Windows systems from XP up to Windows 8. The description below is for XP, but in the newer systems the procedure is similar.
The first thing to do is to see if the start-up command is still present in the registry. Open Regedit and go to HKLM\Software\Microsoft\Windows\Current version\Run. There should be a string named avast of type REG_SZ with a value something like "C:\Program Files\AVAST Software\Avast\avastUI.exe" /nogui. The quotes included. If this string is lacking, there is a good chance that the procedure below will work. If the string is present you may nevertheless try the steps below. Close Regedit.
First of all it is wise to make an image of your C-disk with a program like Macrium Reflect, Norton Ghost or similar.
Then open the Avast user interface and click the "settings" button in the right upper corner of the screen. A window will pop-up. Click "Maintenance" in the left pane and click "Backup settings", then "Continue" and choose a safe place to store the settings file, which has the extension "avastconfig". Close the Avast user interface.
Then open Add/Remove programs from the control panel and click avast!Free antivirus. A pop-up window will open and then choose "Repair". Run the repair routine. When ready close Add/Remove programs.
Now go back to Regedit and see if the string as indicated above is there. If it is where it first was lacking, you probably have solved the problem. Close Regedit.
Go back to Avast>Settings> Maintenance> Backup as described above and restore your settings from the saved file. Now reboot. If the procedure worked for you everything should be back to normal.
Hope it helps.